Method for welding a double layer sheet to a jointing sheet

ABSTRACT

The invention relates to a method for welding a double layer sheet metal ( 1, 2 ) having an intermediate layer ( 5 ), in particular, a sheet metal with knobs, with a bonding sheet metal ( 8 ) by spot or roll seam welding. Before welding of the bonding sheet metal ( 8 ) with the two cover sheet metals ( 1, 2 ), spaced apart by the intermediate layer ( 5 ) of the double layer sheet metal, is carried out, the area ( 11 ) about the welding location ( 12 ) is burned free of the intermediate layer ( 5 ) by the current which is introduced over a large surface area into the cover sheet metals ( 1, 2 ) via the welding electrodes ( 9, 10 ) placed thereat and guided via a shunt adjacent to the welding location ( 12 ). Only thereafter, the sheet metals are brought into conducting contact with one another by a directed local pressure application, and the welding process is carried out.

The invention relates to a method for welding a double layer sheetmetal, comprised of two cover sheet metals and an intermediate layer ofa combustible material, in particular, of impregnated paper, with abonding sheet metal by resistance or resistance roll seam welding,wherein the first cover sheet metal rests against the bonding sheetmetal and one of the oppositely positioned welding electrodes contactsthe second cover sheet metal and the other one of the welding electrodescontacts the bonding sheet metal, respectively, for performing thewelding process, so that the current between the welding electrodesfirst flows via the second cover sheet metal, at least one shuntpositioned adjacent to the welding location, the first cover sheetmetal, and the bonding sheet metal, so that thereby the intermediatelayer is heated and removed in the area of the welding location and,subsequently, the cover sheet metals, with the bonding sheet metal, arewelded together by directly contacting one another.

Double layer sheet metals are known in various embodiments. In a knownmethod for manufacturing such a double layer sheet metal of theaforementioned kind (Patent Abstracts of Japan, vol. 013, No. 060(M-796), Feb. 10, 1989, and JP 63 264279 A; and Patent Abstracts ofJapan, vol. 10, No. 301 (M-525), Jun. 3, 1986, and JP 61 115687 A), oneof the cover sheet metals and the bonding sheet metal are contacted bytwo electrodes which are of identical size and limited to the area ofthe welding location. The shunt is formed by an outer bridge whichcontacts one of the cover sheet metals and the bonding sheet metal faroutside of the welding location. In order to prevent that, when avoltage is applied, the current density at the relatively small contactlocations of the welding electrodes is too great, the first mentionedprior art proposes to affect the current conduction by controlling theapplied voltage such that, first, a current of low current intensityflows which results in melting of the intermediate layer comprised ofresin. As soon as the two cover sheet metals, with displacement of themelted intermediate layer, have been contacted as a result of theclamping force acting on the one cover sheet metal and the bonding sheetmetal, which can be detected by a change of the applied voltage, thecurrent between the electrodes can flow directly via the cover sheetmetals and the bonding sheet metal. From this point in time on, thecurrent is increased to the required welding current intensity and thewelding connection between the bonding sheet metal and the cover sheetmetals is produced. This method thus requires current and voltagecontrol during the bonding process.

In a known double layer sheet metal (DE 195 03 166 A1), of which atleast one cover sheet metal has knobs, the two cover sheet metals arewelded to one another at the end faces of the knobs wherein, however,not all knobs must be welded to the other cover sheet metal. Such doublelayer sheet metals are characterized by a high geometrical moment ofinertia while having a relatively small weight per surface area. Becauseof this property, they are pre-destined to be used in motor vehicles.

Moreover, it is known to fill the hollow space between the two coversheet metals with a filler. This filler can act as a reinforcing meansand as a noise dampening means. The manufacture of such a double layersheet metal with an intermediate layer is carried out such that theintermediate layer is placed as a perforated mat onto the sheet metalwith knobs such that the end faces of the knobs are exposed.Accordingly, after placement of the other cover sheet metal, the twocover sheet metals can be welded together without problems at thedirectly contacting end faces of the knobs by resistance welding.

In contrast, the welding of such a double layer sheet metal with abonding sheet metal by resistance welding is a problem because of theinsulating intermediate layer. Welding of the bonding sheet metal withthe double layer sheet metal in the area of the end faces of the knobscan be carried out without problems, but not on locations where betweenthe two cover sheet metals the insulating intermediate layer ispositioned. Welding experiments on double layer sheet metals with coversheet metals of 0.2 to 0.4 mm thickness by means of conventional weldingelectrodes with spherical contact surfaces, wherein one of the weldingelectrodes contacts directly the sheet metal with knobs and the otherwelding electrode is contacted indirectly via the bonding sheet metalwith the sheet metal with knobs, have shown that, in particular, anannular melt through of the sheet metal with knobs that is contacteddirectly by the welding electrode can occur. A high-strength connectionof the bonding sheet metal with both cover sheet metals of the doublelayer sheet metal can therefore not be achieved.

It is an object of the invention to develop a method which ensures thata bonding sheet metal can be connected by resistance welding with bothcover sheet metals of a double layer cover sheet metal with insulatingintermediate layer.

This object is solved with a method of the aforementioned kind such thatone of the welding electrodes contacts the second cover sheet metal witha surface area that is at least ten times as large as thecross-sectional surface area of the welding location, that the flowingcurrent burns off, without melting it, the intermediate layer in thearea of the welding location by controlled heating of the cover sheetmetal and that, subsequently, after the welding location has beencompletely burned free, both cover sheet metals are brought intoelectrically conducting contact with the bonding sheet metal by adirected local pressure application only at the welding location and arethen welded together therewith.

With the method according to the invention a welding connection of thebonding sheet metal with both cover sheet metals is obtained, because,upon placement of the one welding electrode, as a result of the verylarge contact surface of the one welding electrode, a current densitythat is too great and thus overheating of the cover sheet metals at thewelding location are prevented, without requiring a limitation of thecurrent flowing during the individual phases of the method. The currentflowing in the cover sheet metals is distributed onto a largecross-sectional surface area and results initially only in that theinsulating intermediate layer, which is disruptive for the weldingprocess, is burned off at least in the direct surroundings of thewelding location. Only thereafter a direct contact of the cover sheetmetals at the welding location occurs. Then there is no longer the riskof burning through or of an annular eroding by heat at the contactlocation of the cover sheet metal at the welding electrode. Asufficiently large current conduction for the burning off process ismade possible by the shunt. In the case of a sheet metal with knobs, theshunt may be the knobs to be welded to one another. In a double layersheet metal without such contact-providing knobs, an artificial contactbridge can be produced between the two double layer sheet metals.

There are several possibilities for the purpose of preventing a currentdensity that is too great at the time of positioning the weldingelectrodes. According to a first alternative, it is suggested that forthe large surface area contacting by one of the welding electrodes, aplate-shaped welding electrode or a ring-shaped welding electrode with acentral support pin is used. According to a second alternative solutionit is suggested that a lost sheet metal piece is used between thewelding electrode and the cover sheet metal which is then also connectedby welding. Such a lost sheet metal piece has a also a further advantagebecause it provides reinforcement.

In order to ensure a direct contact of the cover sheet metals afterburning off the intermediate layer, according to one embodiment of theinvention it is suggested that the other welding electrode is a weldingelectrode having a spherical contact surface. When pressure loadingoccurs, the welding electrode with the spherical contact surface pressesvia the bonding sheet metal the cover sheet metal into the hollow spaceof the double layer sheet metal and brings it into contact with theother cover sheet metal.

In the following the invention will be explained in more detail with theaid of an embodiment illustrated in the drawing. It is shown in detailin:

FIG. 1 a schematic representation in axial section of a double layersheet metal and bonding sheet metal, arranged between two weldingelectrodes, at the moment of contacting;

FIG. 2 the double layer sheet metal and the bonding sheet metalaccording to FIG. 1 after the welding process; and

FIG. 3 a double layer sheet metal with bonding sheet metal and a lostsheet metal piece after the welding process in an isometricrepresentation.

The double layer sheet metal illustrated in FIG. 1 is comprised of twocover sheet metals 1, 2 made of steel with a characteristic thickness of0.2 to 0.4 mm. The lower cover sheet metal 1 illustrated in the drawingis flat, while the upper cover sheet metal 2 is provided with knobs 3,4. The two cover sheet metals 1, 2 are welded together at the end facesof the knobs 3, 4. An intermediate layer 5 of impregnated paper in theform of a perforated mat is positioned between both cover sheet metals1, 2. The holes 6, 7 in the intermediate layer 5 correspond with theknobs 3, 4 so that during welding of the two cover sheet metals 1, 2they directly contact one another via the end faces of the knobs 3, 4.For a conventional knob depth of scantly 2 mm and a sheet metalthickness of 0.2 to 0.4 mm, this results in a double layer sheet metalof approximately 2 mm total thickness.

For such a double layer sheet metal 1-7 it is now desired to connect abonding sheet metal 8 made of steel with a characteristic thickness ofbetween 0.5 and 1 mm by resistance welding and, in particular, by spotor roll seam welding, with the double-layer sheet metal 1, 2. Forwelding, the bonding sheet metal 8 is placed flat against the outer sidela of the inner cover sheet metal 1, as illustrated in FIG. 1.Subsequently, the welding electrodes 9, 10 are placed thereagainst fromboth sides. As can be seen, the axes of the welding electrodes 9, 10 arelocated in the area of the insulating intermediate layer 5 and notwithin the area of a knob 3, 4.

While the lower welding electrode 9 has a spherical contact surface 9 awith a ceramic ring or PTFE ring 9 b placed there about, which in thecase of galvanized sheet metal is designed to prevent an annular zincformation on the bonding sheet metal crater, the upper welding electrode10 is a ring-shaped electrode. It has a contact ring 10 a and a centralsupport pin 10 b which is slightly recessed so that, upon placement ofthe welding electrode 10 onto the outer side 2 a of the cover sheetmetal 2 provided with knobs, it does not yet contact, in contrast to thering contact 10 a. As soon as now current is supplied to thisconstellation, this current is distributed over a large surface areainto the cover sheet metals 1, 2 wherein the areas welded to one anotherform a shunt via the knobs 3, 4. For such a contact arrangement andcurrent introduction into the double layer sheet metal 1-7, a verystrong heating of the cover sheet metals 1, 2 results because of theespecially high current density in the direct vicinity of the lowerwelding electrode 9 and thus also in the area 11 of the future weldinglocation 12, which causes the burning off of the insulating intermediatelayer 5 in this area 11 of the welding location 12, which, however, isnot so great that it would cause melting/melting away of, for example,the upper cover sheet metal 2, in this area. Since the weldingelectrodes 9, 10 are loaded by pressure and the pressure loadingcontinues, the lower welding electrode 9 presses with its sphericalcontact surface the bonding sheet metal 8 and the lower sheet metal 1into the free space provided by the burning off of the intermediatelayer 5 so that the lower cover sheet metal 1 can contact directly theupper cover sheet metal 2. In this context, the support pin 10 bsupports the upper cover sheet metal 2. In final analysis, one obtainsin this way a welding connection between the bonding sheet metal 8 andthe double layer sheet metal in which the two cover sheet metals 1, 2are welded to the bonding sheet metal 8.

In the embodiment according to FIG. 3, instead of the ring-shapedwelding electrode 10, a lost sheet metal piece 13 is used. In this case,the same welding electrode as the welding electrode 9 of the embodimentaccording to FIG. 1 can be placed against this sheet metal piece 13.

Inasmuch as double layer sheet metals without knobs are to be connectedby welding with a bonding sheet metal, a shunt can be produced in that,laterally adjacent to the future welding location, an electrical bridgeis connected to the bonding sheet metal which is in connection with theoppositely positioned cover sheet metal.

What is claimed is:
 1. A method for welding a double layer sheet metal(1, 2, 5), comprised of two cover sheet metals (1,2) and an intermediatelayer (5) of a combustible material, with a bonding sheet metal (8) byresistance or resistance roll seam welding, wherein the first coversheet metal (1) rests against the bonding sheet metal (8) and one of theoppositely positioned welding electrodes (9, 10) contacts the secondcover sheet metal (2) and the other one of the welding electrodescontacts the bonding sheet metal (8), respectively, for performing thewelding process, so that the current between the welding electrodes (9,10) first flows via the second cover sheet metal (2), at least one shunt(3, 4) positioned adjacent to the welding location (12), the first coversheet metal (1), and the bonding sheet metal (8), so that thereby theintermediate layer is heated and removed in the area of the weldinglocation (12) and, subsequently, the cover sheet metals (1, 2), with thebonding sheet metal (8), are welded together by directly contacting oneanother, characterized in that the one welding electrode (10) contactsthe second cover sheet metal (2) with a surface area that is at least 10times greater than the cross-sectional surface area of the weldinglocation (12), that the flowing current burns off the intermediate layer(5) in the area (11) of the welding location (12) by controlled heatingof the cover sheet metal (1, 2) without melting it, and that, aftercompletion of burning free the welding location, both cover sheet metalsare brought into electrically conducting contact with the bonding sheetmetal (8) only at the welding location (12) and are welded together withthe latter.
 2. The method according to claim 1, characterized in thatfor the large surface area contacting by the one welding electrode (10)a plate-shaped welding electrode or a ring-shaped welding electrode witha central support pin (10 b) is used.
 3. The method according to claim1, characterized in that for the large surface area contacting by theone welding electrode (10) a lost sheet metal piece (13) is used betweenthe welding electrode and the cover sheet metal (2) which piece is alsoconnected by welding.
 4. The method according to claim 1, characterizedin that the other welding electrode (9) is a welding electrode with aspherical contact surface (9 a).
 5. The method according to claim 1,characterized in that an electrical bridge between the cover sheetmetals (1, 2) and/or the sheet metal piece (13) and/or the bonding sheetmetal (8) is used as the shunt.
 6. The method according to claim 5,characterized in that at least one cover sheet metal (2) with knobs (3,4) is used which are welded with their tips to the other cover sheetmetal (1) and act as a shunt.
 7. The method according to claim 1,wherein the combustible material is impregnated paper.